1. Field of the Invention
The invention relates to interferometers and, more particularly, relates to a geometrically-desensitized interferometer (GDI) instrument for surface profiling. Even more particularly, the invention relates to a GDI instrument that includes an optical assembly configured to redirect and/or suppress stray beams. The invention additionally relates to a method of operating an interferometer with improved stray beam management capability.
2. Discussion of the Related Art
Optical metrology of surface profiles can generally be divided into two regimes, namely interferometric and geometric. Geometric techniques include triangulation and moire fringe analysis, which involves the projection and imaging of a periodic structure such as a ronchi ruling. Geometric techniques are relatively insensitive to surface roughness and deformations, but are of relatively low resolution--so low, in fact, that they are unsuitable for many applications in which surface profiles must be measured with high precision.
Interferometry, on the other hand, relies on the wave nature of light to ascertain with high precision the surface profile of a test object. A typical traditional interferometer includes a light generator that generates a beam of light, a spatial filter-beam diverter that diverts the light beam into a diverging spherical wavefront, a beamsplitter that diverts part of the diverging spherical wavefront from the filtered beam, and a collimating lens that collimates the wavefront to produce a plano wavefront of coherent light. This wavefront of coherent light is then reflected off test and reference surfaces to produce first and second reflected wavefronts which combine with one another while interfering both constructively and destructively to produce an interference fringe pattern. An imaging device such as a solid state camera receives the recombined wavefronts and acquires images of the interference fringe pattern. The interference fringe pattern then is analyzed to obtain information about the surface profile of the test object.
Fringe pattern analysis for surface profilometry often is performed by the well-known technique of phase shifting interferometry (PSI). In PSI, the height difference between locations on a surface imaged by first and second pixels on the imaging device is determined by first determining a phase difference between light received at the first and second pixels and by then using the phase difference to calculate a height difference. A primary advantage of PSI is that it is highly precise. The vertical height precision for PSI is a fraction (e.g., 1/100 ) of the optical wavelength of the light source used to conduct the measurement.
Generally speaking, however, conventional PSI approaches can only profile smooth surfaces having relatively small height variations or "surface departures" between adjacent measurement sites. This constraint results from the fact that PSI has a phase ambiguity constraint. Specifically, the maximum physical departure between adjacent measurement sites on the profiled surface must be less than 1/4 of the source wavelength. Stated another way, the maximum phase difference between the reference and test light beams must have an absolute value which is less than .pi.. This constraint, sometimes known as "two .pi. ambiguity", arises because the arctangent function, which is used to convert phase to distance, is only unique within the range of .+-..pi.. Thus, although the use of phase measurements advantageously allows very high precision to be obtained, it disadvantageously limits the maximum surface departure between adjacent measurement sites to one quarter of the source's optical wavelength. A further difficulty with PSI arises when the surface slope is so large that it becomes difficult to resolve or distinguish the interference fringes because the fringe density is too high. Therefore, while PSI interferometry is much more precise than geometric optical profilometry, it historically has been considered to be ill-suited for use with rough objects or objects having marked surface deformations. Interferometers using PSI analysis therefore historically have not been considered appropriate for some surface profilometry applications.
One interferometric technique that lacks the quarter-wavelength constraint of PSI is the so-called scanning white light interferometry or SWLI. In SWLI, a white light illumination source or, more generally, one which is of a broad-band as opposed to being of a narrow-band generates an interference pattern which contains regions of high contrast for each location on the test surface as a function of scan position. The scan position of high contrast for a given pixel indicates the height of the corresponding location on the test surface. Therefore, by comparing the temporal characteristics of these regions of high contrast with one another, a difference in height between two locations on the profiled surface can be determined. Unlike PSI, SWLI does not calculate height differences based on phase differences, and the PSI phase constraint therefore does not apply to SWLI. The maximum physical departure between adjacent measurement sites on a profiled surface therefore may be much larger with SWLI than with PSI.
However, SWLI has disadvantages of its own that hinder its use in industrial applications. For instance, the field of view is generally no larger than can be accommodated by standard microscope objectives. To function correctly, the imaging device of the instrument must have high resolution when compared to the corresponding interference fringe density. When the field of view of the typical SWLI instrument is increased, the fringe density can easily become difficult to resolve even with very high resolution imaging devices. This problem is especially evident during the profiling of rough surfaces. Moreover, slope tolerance for specular surfaces decreases linearly with the field size, and the speckle effects required for rough-surface measurements are only resolvable if the numerical aperture (NA) of the objective decreases linearly as the field increases. The need to resolve the speckle pattern from rough surfaces is the most discouraging, since the amount of collected light decreases with the square of the NA. The light loss means that larger surfaces require a more powerful illuminator. Worse, the fringe contrast is now a highly variable parameter, and the quality of the measurement depends critically on the balance between the reference and object beam intensities.
Another disadvantage of typical SWLI techniques is that data acquisition is very slow. The slow speed is a consequence of the rapidly varying interference effect as a function of scan position. Accurate measurements require that these variations be recorded in detail, usually at the rate of one measurement per pixel per 75 nm of scan motion.
Still another disadvantage of typical SWLI is its high sensitivity to vibration. An instrument configured for SWLI analysis generally requires massive mounting fixtures and expensive vibration isolation.
Recent years have seen an increased demand for the high speed, high precision metrology of the surface profiles of manufactured parts having large surface departures, i.e., having rough surfaces or surfaces with pronounced surface deformations. A corresponding demand has arisen for the acquisition of data during production as opposed to in the laboratory. For instance, precision products such as hard disks for computer disk drives need to be profiled with high precision, at high speeds, and under conditions in which the test object may be subjected to substantial vibrations during manufacturing processes. Neither traditional PSI techniques nor traditional SWLI techniques are suitable for these purposes. A need therefore has developed for a "desensitized" interferometer that is relatively insensitive to surface roughness and surface deformations, that performs surface metrology with high accuracy and at high speeds, and that is relatively insensitive to vibrations and therefore is well-suited to production-line use.
This need has been met to a large extent by the development of the geometrically-desensitized interferometer (GDI) instrument. As is discussed in an article by de Groot entitled "Grating interferometer for flatness testing" Opt. Lett. 21(3) 228-230 (1996), a GDI instrument is characterized by the replacement of the beam splitter of the traditional instrument with an optical assembly located between the collimating lens and the test object. The optical assembly, which typically (but not necessarily) comprises a diffraction grating assembly, a hologram, or diffractive optics in combination with conventional optics such as mirrors and lenses, divides the collimated source light into two beams which propagate in two different directions and impinge on the profiled surface at the same location but at different incident angles. The beams reflect from the profiled surface and pass back through the optical assembly in different directions, after which they are recombined. Constructive and destructive interference of the reflected and recombined beams form an interference fringe pattern having an equivalent wavelength that may be orders of magnitude larger than the source wavelength. As a result, the GDI instrument is much less sensitive to height variations and surface deformations than are traditional interferometers using PSI analysis techniques. Many disadvantages associated with SWLI such as a limited field of view, a slow acquisition speed, and a high sensitivity to vibration therefore are avoided. The sensitivity of GDI instruments is intermediate between conventional interferometry and moire fringe analysis, and is comparable to that obtained with grazing-incidence interferometry. GDI instruments therefore can be used in manufacturing applications and other applications that are unsuitable for traditional interferometry.
A characteristic of GDI instruments that has not heretofore been fully addressed is that they lack fully-effective stray beam management measures. "Stray beams" are beams of light that propagate from the optical assembly of the GDI instrument to the pupil of the instrument's CCD camera or other imaging device and that degrade the instrument's effectiveness. Stray beams can be divided conceptually into two categories, namely 1) "back reflections" and, 2) ghost images. Each of these two types of stray beams and their attendant problems will now be discussed.
Back reflections result from the multiple-reflection and diffraction of light within the diffraction gratings or other components of the optical assembly of the instrument, and are present even in the absence of a test object. In the usual case in which the optical assembly comprises a pair of diffracting gratings, back reflections may result, e.g., from reflection or diffraction on the various air-glass interfaces, imperfections in the grating's substrate, sharp edges on the grating's groove profile, etc. For instance, and as illustrated in FIG. 8, stray light may propagate from the coarse grating G1 of a typical GDI instrument by reflecting from the back surface of its substrate as rays R1, by reflecting from the fine grating G2 itself as rays R2, or by reflecting from the back of the fine grating substrate as rays R3. The back reflections are superimposed on the properly-propagating reflected beams A', B' as noise. This noise can significantly reduce the accuracy of GDI measurements, particularly when the test object surface reflects light poorly and, accordingly, produces only a relatively weak interferometric measurement signal with a correspondingly high signal-to-noise ratio.
Attempts have been made to ameliorate the effects of back reflection on a GDI instrument by introducing wedge and/or tilt into the diffraction gratings or other optical elements of the instrument's optical assembly so as to direct back reflections away from the pupil of the instrument's CCD camera or other imaging device. However, heretofore, directing stray beams away from the imaging device pupil have complicated the setup and alignment of the instrument. In order to achieve the best measurement accuracy, the test surface must be positioned in space near to an optimum metrology plane. The optimum metrology plane is defined as that ideal test surface for which beams reflecting at different angles from any single surface location on the ideal test surface recombine and impinge upon the instrument's imaging device at a single location. The introduction of wedge and/or tilt into the diffraction gratings can result in a marked phase offset when the test object surface is properly positioned near the optimum metrology plane. Phase offset is an optical phase difference proportional to the difference in optical path length of the beams reflecting from a given point on the test object surface. When the instrument employs an extended light source, phase offsets can result in significant displacements of the location of maximum interference fringe contrast from the optimum metrology plane. The net result is that the position in space corresponding to optimum fringe contrast is not always coincident with the optimum metrology plane. Depending on the particular configuration of the instrument, it is also possible that the phase offset changes linearly across the field of the instrument. For convenience we thus separate the overall effect in two contributions: 1) an average phase offset, measured as a distance L in the Z direction from the optimum metrology plane; 2) a phase offset variation expressed as the number of tilt fringes appearing in the field of the instrument when the object is at the optimum metrology plane.
As a result of the phase offset referred to in the previous paragraph, it was heretofore necessary to first translate the test object to locate the position of maximum fringe contrast, and to then translate the test object an additional preset amount in order to locate the position of the optimum metrology plane. This additional object translation requires considerable skill on the part of the operator and complicates instrument operation. An additional complication is that the fringe contrast is generally poor when employing an extended light source when the test object is properly positioned within the optimum metrology plane.
A proposal has been made to improve a GDI instrument's ability to measure a variety of surfaces with both high contrast and high accuracy. Specifically, U.S. patent application Ser. No. 09/003,449 to de Groot (the de Groot application), filed Jan. 6, 1998 and entitled "Geometrically-Desensitized Interferometer with Adjustable Range of Measurement Depths," proposes a GDI instrument having a variable-geometry light source that can be manipulated to switch the instrument between 1) a low coherence operational mode, in which the instrument has an extended source and consequently a small coherence envelope, and 2) a high coherence operational mode, in which the instrument has a narrow source and consequently a large coherence envelope. The low coherence operational mode is employed in the setup of the instrument to assist in the location of the optimum metrology plane. Once this plane is identified, the system is switched into high coherence mode to improve fringe contrast for all subsequent measurements. Although the ability to switch between coherence modes facilitates the proper operation of the instrument, the de Groot application does not teach how to eliminate phase offsets at the optimum metrology plane and in this way eliminate the need to switch between coherence modes every time a test object is aligned for measurement.
Turning now to the issue of stray beams that can cause ghost images, these ghost images are caused by stray beams coming from the object through the system in an undesired or unanticipated manner, resulting in duplicate images that can overlap the desired image and degrade its quality. These object ghosts can be understood in conjunction with FIG. 9, which schematically illustrates a GDI instrument in which the gratings G1 and G2 of the optical assembly are designed to transmit only a first diffraction order A of -1 and a second diffraction order B of +1. Propagation of other diffraction orders such as the undesired 0.sup.th diffraction order C produces ghost images of the test object surface in the interference pattern that are identical to but spatially offset from the true images produced by useful diffraction orders. In a GDI instrument having inadequate diffraction order management capability, an incident beam generates a number of additional diffraction rays both in transmission and in reflection when it strikes a grating and generates diffraction rays of useful orders.
It is therefore understood in the art that proper GDI design requires suppression of the unwanted diffraction orders (see for example the aforementioned article by de Groot, entitled "Grating interferometer for flatness testing"). However, suppression of the principal unwanted diffraction orders, such as the 0.sup.th diffraction order shown in FIG. 9, generally leads one to employ rectangular or triangular (also known as "blazed") groove profiles. These profiles can generate weak, high-order diffracted beams because of their sharp edges. The term "high order," as used herein, refers to beams of third order and higher. Even though these high diffraction orders are weak because of low diffraction efficiency, some of them have very high angles of incidence on the AR coated surface of the substrate. They are therefore reflected very efficiently back towards the imaging device of the instrument. Some of these diffraction orders even undergo total internal reflection, resulting in comparatively bright stray light paths that contribute to back reflections. The worst case is when the gratings have plane, parallel substrates that are themselves parallel to the object plane. In this case, many of the multiply-reflected and diffracted rays inside the grating assembly are parallel to the optical axis of the imaging device. As a result, the added contributions can actually flood the detector if the source power is high. Thus, it is seen that one possible consequence of suppressing ghost images is the generation of additional unwanted back reflections.
The need therefore exists to design the optical components of a GDI instrument so that stray beams either are not generated or are deviated away from the pupil of the imaging device.